1. Field of the Invention
The present invention relates to a TFT substrate and a method of manufacturing the same.
2. Description of Related Art
A liquid crystal display device for display using liquid crystals is widely applied to products having features of low power consumption and low profile type as one of flat panel displays as an alternative to a cathode ray tube (CRT).
A simple matrix liquid crystal display device and an active matrix liquid crystal display device are among types of the liquid crystal display device for display using liquid crystals. The active matrix liquid crystal display device is a TFT-LCD that uses a thin film transistor (TFT) as a switching element, for example. The TFT-LCD has features superior to the CRT or the simple matrix liquid crystal display device in terms of portability and display quality, and it is widely applied in practice to notebook personal computers or the like. In the TFT-LCD, a liquid crystal layer is generally placed between a TFT array substrate and a counter substrate. TFTs are arranged in an array on the TFT array substrate. Polarizing plates are respectively placed on the outer side of the TFT array substrate and the counter substrate, and a backlight is further placed on one side. In such a structure, suitable color display can be obtained.
However, when manufacturing the TFT array substrate used in the TFT-LCD, it is necessary to form TFTs in an array on a glass substrate by using semiconductor technology, thus requiring a large number of manufacturing steps. Therefore, various defects and failures are likely to occur to cause a decrease in yield, and a large number of devices are required for manufacture to cause an increase in manufacturing cost.
As a method frequently used hitherto as a manufacturing method of a TFT array substrate for LCD, Japanese Unexamined Patent Application Publication No. 8-50308 discloses several manufacturing methods using five-time photolithography processes (5 mask processes) and structures thereof.
Further, a method of manufacturing a TFT array substrate by four-time photolithography processes (which is referred to hereinafter as 4 mask process) by using halftone exposure technology on the basis of the basic structure described in the seventh embodiment (paragraphs 0083 to 0089 and FIGS. 54 to 63) of Japanese Unexamined Patent Application Publication No. 8-50308 is disclosed in Japanese Unexamined Patent Application Publication No. 2005-283689 (paragraphs 0019 to 0025 and FIGS. 14 to 15) and Japanese Unexamined Patent Application Publication No. 2001-339072. In the 4 mask process, the film thicknesses of photoresist are varied by using the halftone exposure technology. The process then forms a semiconductor active layer, source and drain electrodes and a channel region of a TFT by using the resist having different film thicknesses. Thus, two-time (second and third) photolithography processes can be collectively performed as one-time photolithography process.